Aurivillius相铋层状结构多铁性晶体的生长及磁电效应研究

Multiferroic materials, especially those characterized by the coexistence of ferromagnetic and ferroelectric orders, have great potentials in the fields of information storage, novel microwave applications, and multifunctional electrotonic equipments etc. They can be divided into two categories from the viewpoint of components, one is the single phase material with intrinsic multiferroic property; the other is the composite material - by coupling between different phases, which could produce strong ME effect through the single component. In this project we are proposing the multiferroic materials with microscopic nano-composite structure - Aurivillius phase bismuth-layer-structured compounds. Such compounds have a natural superlattice structure， which will contribute to ferroelectricity and magnetism.The crystals will be grown using micro-floating zone method. The containerless solidification process using an aerodynamic levitator and laser heating will be used to characterize the properties of melting heat, heat conduction, radiation heat, density, and the bismuth's volatilization, as well as the surface tension and viscous coefficients and so on.The Shanghai synchrotron radiation imaging combining levitating technology will be used to in situ observe the solidification and crystalization process. It is expected to capture the the dynamic behavior, as a result, the formation mechanism will be useful to optimize the technology of crystal growth and finally obtain good crystals. Focusing on the emphasis of the relationship between the structure and properties will reveal the mechanism of ME coupling. It is therefore of great importance to provide basis for optimizing the design and development of the multiferroic functional devices.

兼具铁电性和磁性的多铁性材料在信息存储、微波、多功能电子设备等领域拥有很大的发展潜力。多铁性材料从组成上可分为两类，一类是具有本征多铁性的单相材料；另一类是复合材料，通过不同组成相之间的耦合可以产生强的磁电效应。本项目提出具有微观纳米复合结构的Aurivillius相铋层状结构多铁体，其独特的超晶格结构将贡献于铁电性和磁性，可以期待本征多铁性和强的磁电耦合。采用微浮区法生长晶体，利用无容器悬浮技术表征晶体熔液的熔解热、热传导、辐射热、密度、铋挥发度、表面张力及粘性系数等性质，采用上海光源与悬浮技术结合对晶体生长进行实时观察，探明其凝固结晶的动力学行为，进而优化生长工艺得到优质单晶。研究它们的微观结构与宏观性质的调控，最终揭示其磁电耦合效应的机理，为多铁性功能器件优化设计和开发提供依据。

多铁性材料从组成上可以分为两类，一类是单相材料，这类材料具有本征的多铁性；另一类是复合材料，通过不同组成相之间的某种耦合作用产生强的磁电耦合效应，然而其单相本身并不具有磁电效应。目前迫切需要寻找室温以上具有较大磁电耦合效应的多铁性材料，更重要的是，通过对本征单相多铁性材料的研究，解决铁电性和磁性共存的微观机制及相互调控的问题。本项目研究了一种“超构材料”：Bi4+nTi3FenO12+3n, 通式还可以改写成Bi4Ti3O12+nBiFeO3 (n=1，2，3，5), 包括Bi5Ti3FeO15 (n=1) （BTFO15）, Bi6Ti3Fe2O18 (n=2)（BTFO18）, Bi7Ti3Fe3O21 (n=3)（BTFO21）, Bi9Ti3Fe5O27 (n=5)（BTFO27）。其中Bi4Ti3O12是性能优异的无铅铁电材料；随着n的增大，化合物中的Fe量增加，期待形成-Fe-O-Fe-长链，即铁的长程有序。不同层数的铋层状结构化合物沿c轴方向交替共生，形成一种天然的超晶格结构，共生形成的铋层状材料可以具有更好的铁电性。通过微浮区法成功生长得到BTFO15, BTFO18, BTFO21和BTFO27四种晶体，结构决定性质，在层内具有更大的漏电流现象，根据晶体结构和各向异性，研究了c轴方向和110方向的性质，c轴显示更好的铁电性，因为垂直于层方向。通过铁电性和磁性的表征证明他们具有室温铁电性和室温磁性，利用压电磁力显微镜在室温下观测到了电畴和磁畴的耦合效应，证实四种晶体是室温下具有磁电耦合效应的单相多铁性晶体。对BTFO15进行了磁电耦合系数的测试，在H = 0时得到了 400 mV/(Oe·cm) 的磁电耦合系数，远远大于文献中报道的数值：0.1 mVcm-1Oe-1 和8.28 mVcm-1Oe-1（陶瓷样品）。 这是因为对比陶瓷样品，晶体和薄膜材料具有更高的质量和更完美的超晶格结构。同时，开创性的采用机械力来进行畴的反转研究，即实现了多态耦合，采用电、磁、机械力实现了畴的多态耦合效应。室温下机械力、电极化和磁化的调控效应，揭示了铁电序和磁序，即自旋翻转和铁电反转的协同进行，预示着量子调控在多铁性体系中的初步实现，具有重要的意义。